Detect head motion (CTF)

1

Hi everyone,
I have a question about the Detect head motion (CTF) function in Brainstorm.
My head-displacement plot shows a maximum displacement of about 4.8 mm.
So with a 5 mm threshold, I expected one single stable-head segment.
Instead, Brainstorm marks several bad segments.

Can someone explain how the detection works and why multiple bad segments appear even though the displacement never exceeds the threshold?

head
head1378×449 58.7 KB

heads
heads375×301 32 KB

2

The algorithm works by comparing the positions to the start of a segment, so it might still go over 5 mm from that point, even if all points are within 5 mm of another reference position.

Imagine if the head moved almost 10 mm in a straight line. The reference might be in the middle and keep everything within 5 mm, but compared to the start, you eventually go over 5 mm.